This application is directed to catalysts suitable for the production of high quality hydrocarbon fractions by cracking high boiling hydrocarbons.
Catalytic cracking is employed in petroleum refining operations to convert gas oils to higher valued hydrocarbons such as gasoline and distillates, e.g., naphtha and kerosine. These gas oils may come from a number of sources such as atmospheric distillation columns, vacuum distillation columns, cokers and recycle streams from the catalytic cracker. The properties of these gas oils vary with the sources from which they come.
In atmospheric distillation, crude oil is subjected to temperatures of up to about 650.degree. F. (343.degree. C.). Higher temperatures tend to thermally degrade the hydrocarbons in the crude oil feedstock. From the atmospheric distillation columns the following fractions may be obtained: a gasoline fraction having a boiling point of less than about 300.degree. F. (149.degree. C.); a naphtha fraction having a boiling range of between about 300.degree. F. (149.degree. C.) and about 360.degree. F. (182.degree. C.); a kerosine fraction having a boiling range of between about 360.degree. F. (182.degree. C.) and about 460.degree. F. (238.degree. C.); at least one gas oil fraction having a boiling range of between about 460.degree. F. (238.degree. C.) and about 650.degree. F. (343.degree. C.); and an atmospheric residua fraction having a boiling point of greater than 650.degree. C. (343.degree. C.). The atmospheric residua may comprise a substantial portion, e.g., from about 30% to about 50% by volume, of the whole crude which is subjected to atmospheric distillation.
As mentioned previously, subjecting the hydrocarbons in a petroleum feedstock to temperature of greater than 650.degree. F. (343.degree. C.) tends to result in the thermal degradation of hydrocarbon. However, one or more relatively heavy gas oil fractions can be separated from atmospheric residua by subjecting this atmospheric residua to vacuum distillation, whereby higher boiling fractions are obtained. In this way, one or more gas oil fractions having a boiling range between about 650.degree. F. (343.degree. C.) and about 1000.degree. F. (538.degree. C.) may be obtained. The vacuum residua having an initial boiling point of greater than about 1000.degree. F. (538.degree. C.) may comprise up to about 25% by volume, e.g., from about 10% to about 25% by volume of a readily refinable whole crude.
Larger, high-boiling temperature hydrocarbons may be converted to smaller, lower-boiling hydrocarbons by cracking processes. Such cracking processes include thermal cracking, which takes place in the absence of a catalyst, and catalytic cracking, which takes place in the presence of a catalyst. Gas oils, which have been separated from petroleum residua, when derived from clean, readily refinable whole crude, tend to be excellent feedstocks for catalytic cracking. However, petroleum residua, including atmospheric residua and, especially, vacuum residua, are far less suitable for inclusion in catalytic cracking feedstocks for a number of reasons. First, such residua fractions contain a large proportion of very large hydrocarbon molecular species, e.g., having boiling points of greater than about 1000.degree. F. (538.degree. C.). Such large hydrocarbon molecules, particularly polynuclear molecules, may tend to be less reactive and may tend to be too large to make effective contact with the active sites of the cracking catalyst. Also, the refining process tends to concentrate, into the residua, species contained in the whole crude which tend to poison the cracking catalyst or otherwise interfere with the catalytic cracking process. Such species include nitrogen, metals such as nickel and vanadium, and carbon residue as measured by CCR, wt. %. In view of these factors, residua fractions tend to be avoided as feedstocks or components thereof for catalytic cracking processes. Instead, residua fractions, particularly vacuum residua, tend to be processed by thermal cracking, typically in a coker.
In accordance with such coking operations, various liquid, thermally cracked products are obtained including coker gasoline and higher boiling fractions. The liquid fraction which has the highest boiling point is a coker gas oil fraction which may have a boiling point of, e.g., from about 450.degree. F. (232.degree. C.) to about 850.degree. F. (454.degree. C.). This coker gas oil may be used as a feedstock for catalytic cracking along with other gas oils from atmospheric and vacuum distillation.
As mentioned previously, when relatively clean, readily refinable whole crudes are refined, the gas oils obtained therefrom can readily be further processed by catalytic cracking. However, when inferior crudes, having a particularly large portion of nitrogen, metals, carbon residue and/or large hydrocarbon molecules, are refined, these species which adversely affect catalytic cracking tend to be carried over in large concentration into even the gas oil fractions as well as the residua fractions of the crude. Such inferior crudes include natural petroleum from certain locations as well as certain synthetic crudes derived from coal, tar sands and shale oil. Gas oils derived from such inferior crudes may not be suitable for use as feedstocks for catalytic cracking.
There is a need for catalysts suitable for cracking processes for converting residua and gas oil fractions which contain large hydrocarbon molecules and/or species found in residua which tend to poison cracking catalysts or otherwise adversely effect the catalytic cracking process.